Electrical Appliance working on alternating current

The magnitude of current of alternating current varies with time
The power developed by the electrical appliances, lets say an electric fan, also varies with time
But why the fan can still operate uniformly?
Or the AC is changed to DC in the electric fan?

True, alternating current varies with time. That is what defines AC. In the USA it alternates sixty times per second. A light bulb's filament does fluctuate slightly in output intensity sixty times every second. But, you don't see it because it happens so fast! The same applies to an electric fan: you may study how an AC motor works. The fan does NOT convert the AC to DC. The end result is the fan appears to us to be blowing at a steady, fixed rate.

Old-fashioned TV sets had a "refresh rate" which meant the picture was being displayed once, erased, and then displayed again...at over thirty times per second. Your eye's persistence made it seem continuous. A spinning disc with a slot cut out of it allowed us to "stop action" this refresh rate.

The magnitude of current of alternating current varies with time
The power developed by the electrical appliances, lets say an electric fan, also varies with time
But why the fan can still operate uniformly?Or the AC is changed to DC in the electric fan?

Some motors require DC to make them work and the direction they turn is determined by which way round you connect the supply AC will just make them vibrate backwards and forwards at the mains frequency. If they have a permanent magnet in them, then this is the case and you need a power supply off the mains or a battery to work them. Most of the motors in mains operated hand tools etc. use an electromagnet, which uses the mains supply too. The motor will go in the same direction, irrespective of the instantaneous polarity of the mains waveform and many of them will also work on DC. It is true to say that the torque on a normal AC 'Brush Motor' does vary in time (as you suggest) - partly because of the AC variation but also because of the angle of the armature coil as it turns. But they are usually made with 'multiple poles' which smooths out these pulses so they do not affect the operation.
There is a third type of motor - and Induction Motor, which requires an AC supply for it to work. Pretty much all really big motors are Induction Motors because they are very efficient and many Induction Motors work off a Three Phase mains supply and they have constant torque throughout the cycle.